Brushless DC motors are well known and are used in numerous applications. For example, brushless DC motors are commonly used to power fans, such as may be found within computers to cool components, are used in hard disk drives, CD players, and electric tools. A brushless DC motor typically includes a stator, comprising one or more windings (e.g. of wire) and a rotor comprising one or more permanent magnets. The rotor may, for example, comprise a ring magnet, or an annular array of magnets. The rotor may be arranged to rotate within the stator, or alternatively the rotor may be arranged to rotate around the outside of the stator.
To operate a brushless DC motor, current is passed through the stator windings, and a magnetic field is generated which interacts with the rotor so as to cause relative rotation between the stator and the rotor. Rotor rotation is controlled by controlling the current in the or each, stator winding in an appropriate manner. In the case of single phase brushless DC motors, comprising a single phase stator winding, this control involves the repeated excitation of the winding with current first in one sense, and then in the opposite sense. In the case of multiple phase windings, rotation may be achieved by arranging for the windings of the different phases to be successively excited, in effect to produce a rotating magnetic field with which the rotor interacts. This control of current in the windings of a DC brushless motor to achieve rotor rotation is known as commutation, and in general involves a periodic switching of current from one current path through the winding(s) to another. This switching may comprise the reversal of current direction through a winding and/or the switching of current path from one winding to another.
A further understanding of the operation of brushless DC motors will be obtained from the following discussion.
In general, the field generated by the excitation of the stator windings may be considered to comprise one or more pairs of North and South poles. This generated field interacts with the magnetic rotor, with each rotor pole being attracted to opposing stator poles and repelled by similar stator poles. As the stator is held steady, the effect is that the rotor rotates with respect to the stator. The speed of rotation of the rotor may be readily varied by controlling the magnitude and the timing of the current passing through the stator.
As the rotor rotates relative to the stator, opposite poles of the rotor and the stator are brought into alignment. In the case of single phase motors, it is then necessary to reverse the direction of current flow within the stator windings, such that the poles of the stator swap over, in order to allow the rotor to continue to rotate. As the rotor rotates yet further, the direction of current flow in the stator windings must be reversed yet again. Hence, for continued rotation, commutation of a single phase brushless DC motor comprises a periodic reversing of the direction of current flow through the stator windings. Thus, commutation is cyclical.
For a single phase motor, a single commutation cycle comprises a first drive portion, in which current is driven through the windings in a first direction, and a second drive portion, in which current is driven through the windings in a second, opposite direction. In this case, the length of the full commutation cycle is defined as the interval between the beginning of one first drive portion and the beginning of the next first drive portion, or, equivalently, as twice the interval between successive changes in direction of the current flowing within the stator windings.
For commutation to be effective, the motor must also comprise means for detecting the relative position of the rotor and the stator in order that the change in current direction occurs at the correct point to ensure continued rotation of the rotor. The position detection is typically achieved using a Hall effect magnetic sensor device (or a number of such devices), which generates an output signal indicative of the distance between the sensor and the nearest pole of the rotor. Other forms of rotor position detection may, of course, be used.
A brushless DC motor is typically operated using a switching circuit, for supplying the current to the stator windings, and a controller to control the switching circuit. The switching circuit and/or the controller may be comprised in the motor itself, or may be separate items. For single phase brushless DC motors the switching circuit is typically an H-bridge circuit arranged between positive and negative (or ground) power supply rails. Winding current direction and timing is controlled by appropriate control of the switching elements within the H-bridge. The controller is typically fabricated as an integrated circuit, though may alternatively be formed from discrete components. The controller has inputs derived from the position detector (e.g. Hall sensor) to sense the position of the rotor, and control inputs to set parameters such as motor speed and direction. The controller has outputs, which supply switching signals to control the switching elements of the H-bridge.
In prior art commutation methods, for single phase motors during the first drive portion of each commutation cycle an average voltage is applied across the stator windings (using PWM techniques, for example) in a first sense, causing a current to flow within the windings in a first direction. As the stator and rotor poles come close to alignment the drive portion ends and the applied voltage is removed, in effect to switch the current “off”. The timing of switch off is determined by the signal from the position detector. There may be a short commutation delay before the second drive portion, in which the same average voltage is applied across the stator windings (if constant rotor speed is required) but in the opposite sense. This causes a current to flow within the windings in the opposite direction. The commutation delay is to ensure that at the point at which the stator and rotor poles pass each other, substantially no current is flowing within the stator windings. This is important to ensure that slight inaccuracies in the timing of the commutation do not cause the motor to slow due to the stator and rotor poles being swapped too soon. The duration of the commutation cycle is equal to the sum of the durations of the first and second drive portions and the two commutation delays (i.e. the delay between the first and second drive portions of one cycle, and between the second drive portion of one cycle and the first drive portion of the next cycle).
The average voltage may be applied across the stator windings in the first and in the second, opposite sense by determining which switching elements within the H-bridge are open and closed. During each commutation cycle, at each moment of current “switch off” (i.e. at the end of the first drive portion and at the end of the second drive portion) typically all switching elements within the H-bridge are opened to interrupt current flow from the supply rails through the stator windings. In other words, current drive to the windings is removed (i.e. it ceases). This switch state is maintained during the commutation delay.
However, it will be appreciated that at these “switch off” points large currents are flowing through the windings. Thus, when all switches are opened (i.e. to remove the applied voltage) a large back EMF is generated (i.e. a large voltage spike is developed across the windings). This large voltage spike can in turn give rise to a large and undesirable current spike. The magnitudes of these voltage and current spikes may be many times greater than the average values of drive voltage and winding current experienced during each commutation cycle.
The problem of the large back EMF and the consequent current spike is exacerbated by the fact that even when the average voltage applied across the stator windings is constant during a drive portion of the commutation cycle, the current within the stator windings tends to rise towards the end of the drive portion of the commutation cycle. This rise in the stator current is due to the change in the inductance of the stator windings associated with the changing relative position of the rotor.
These large voltage and current spikes induce vibration in the motor as the stator windings and the rotor magnets vibrate in sympathy with the changes in energy. This vibration causes audible clicks, which is usually undesirable. Additionally, electrical noise may be generated on the motor voltage supply that can be damaging to other equipment, such as CPUs that share the same power supply. In certain arrangements, the electrical noise on the voltage supply is a result of current passing through the parasitic body diodes of the transistors that form the switching elements (or any external diodes present) within the switching circuit. These body diodes act as charge pumps, raising the voltage on the supply rail temporarily higher than its normal level. In order to prevent the voltage spike on the supply rail from damaging connected equipment it is known to isolate the brushless DC motor via a blocking diode, arranged on the positive power supply rail, such that current may flow from the power supply network to that part of the supply rail local to the motor, but not in the reverse direction during locally generated voltage spikes.
The size of the current and voltage spikes at the switching points in the commutation cycle are dependent on the magnitude of winding current at these points. They are, therefore, partly dependent upon the timing of these switch-off points. If current switch-off is done earlier in the cycle, i.e. when the poles of the rotor and the stator are further apart, then the sizes of the spikes can be reduced. This is because, as described above, the stator current tends to rise towards the end of the drive portion of the commutation cycle due to inductance change caused by the changing position of the rotor relative to the stator. By switching off earlier, excessive rises in winding current can be avoided. The switch-off timing (i.e. the timing of the removal of the applied voltage) may conveniently be varied by moving the position of a Hall sensor, arranged to detect rotor position, around the circumference of the stator.
However, removing the applied voltage earlier necessarily results in an increase in the commutation delay, otherwise the stator poles will be switched over before the rotor poles have passed causing rotation of the rotor to be resisted. If this occurs the motor may slow or even stop due to the rotor not having sufficient inertia to rotate past the position in which the poles are aligned. A side effect of increasing the commutation delay is that the proportion of time during each commutation cycle for which the motor is not being powered is increased, resulting in a decrease in speed, which must be counteracted by supply of a greater current to the stator windings throughout the rest of the cycle. Additionally, the rate of rotation of the rotor will vary in an uncontrolled manner throughout each commutation cycle.
The large current and voltage spikes may also physically damage motor components, in particular the switching elements. A known technique to address this problem of large currents and voltages is to use components having higher voltage and current ratings than the maximum expected peak values at the end of the drive portion(s) of each commutation cycle. However, these components, notably transistors, are therefore rated for significantly higher voltages and currents than is required for the remainder of the commutation cycle. It is undesirable to use over specified transistors as the internal resistance loss is increased by the use of higher voltage components, which therefore leads to energy being wasted. Additionally, the cost of electronic components typically increases as the voltage and current ratings increase, resulting in a more expensive motor.
The torque generated within a brushless DC motor is inversely proportional to the square of the distance between opposing poles of the rotor and the stator. Additionally, the torque is proportional to the size of the current passing through the stator windings, as this affects the magnitude of magnetic flux density generated within the electromagnet. Towards the end of each drive portion of each commutation cycle, when the winding current rises due to inductance change as discussed above, the opposing poles of the rotor and the stator come close together. Consequently, this winding current rise generates little torque, and may therefore be considered to be wasted energy.
The current spike due to the back EMF may also be considered to be wasted energy. A partial solution to the problem of wasted energy is to provide a large capacitor across the voltage supply to the H-bridge, such that at the end of each drive portion of the commutation cycle when the back EMF of the coil creates a large voltage spike across the capacitor this excess voltage charges up the capacitor, storing energy to help power the next commutation cycle. However, using a capacitor to store electrical energy is inefficient because in order to charge the capacitor the charge current must pass through the body diodes of the transistors forming the switching elements (or external diodes if these are present). It is preferable not to have to try to recover this energy in the first place. Additionally, due to the large value capacitance required the capacitor may be physically large. There may not be physical space within a motor housing for the capacitor. Consequently, the result is a compromise between a medium sized capacitor and accepting some voltage spike on the voltage supply to the H-bridge, necessitating some over specifying of components. Also, this approach does not have any impact on the problems of over specified components and acoustic/electrical noise as described above.
When the supply voltage is applied across the stator windings at the beginning of each drive portion of each commutation cycle the current flowing through the coil builds steadily to an early peak. This gradual rise is due to the inductance of the stator windings. The motor is most efficient during the early part of each drive portion as the opposing stator and rotor poles are further apart. Consequently, this early peak represents the most efficient part of the commutation cycle.
A further known method of reducing the size of the voltage and current spikes at the end of each drive portion of the commutation cycle is to limit the maximum current that may flow through the stator windings. This has the effect of flattening the current profile throughout the whole drive portion. However, while this method does remove the worst effects of the voltage and current spikes in terms of noise and damage to components, this method is inefficient. The unwanted current spike cannot be limited to a current level lower than the pulse at the beginning of each drive portion without unduly limiting that part of the cycle also. Therefore, the best that can be achieved with this approach is a slight flattening of the current waveform over the whole of the commutation cycle. As discussed above, the early part of the drive portion of the commutation cycle, when the like poles are closest together, provides the greatest torque for a given current passing through the stator windings. Therefore, it is desirable not to limit the current flow during the early part of the commutation cycle, whilst addressing the problem of voltage and current spikes generated at the ends of the drive portions of the commutation cycle. This cannot be achieved with basic current limiting techniques.